This invention relates generally to a source of diffused illumination, and more particularly to a backlight for an electrooptic display. Diffused illumination devices have been used for many years to scatter and diffuse the light from a point source, such as a miniature incandescent bulb so as to illuminate dials at night or in dim light. These typically provide means to capture the light at the edge or at the end of a light transmitting member or a "light pipe" of transparent material such as glass or transparent plastic, and to guide the light and scatter and diffuse it with suitably disposed reflectors and light scattering elements. Illustrative of such devices are U.S. Pat. Nos. 3,043,038 issued July 10, 1962 to Marble, 3,043,947 issued July 10, 1962 to Albinger and 3,574,993 issued Apr. 13, 1971 to Black. These patents show light emanating from a point source, conducted through a light pipe, and scattered by irregularities at the face, with reflectors on the back surface. In addition, Marble and Black include transluscent sheets on the front face to diffuse the light.
Backlights have more recently been used for electrooptic displays, such as liquid crystal displays, which can operate both in a transmissive and reflective mode. Thus, the backlight may include an element designed to serve as a reflector of ambient light, as well as providing illumination for the display when ambient light is insufficient. Exemplary of such construction patents are U.S. Pat. No. 3,864,905 issued Feb. 11, 1975 to Richardson, British Pat. No. 1,417,958, U.S. Pat. No. 4,043,636 issued Aug. 23, 1977 to the applicant U.S. Pat. No. 4,118,111 to Laesser and pending application Ser. No. 935,187 filed Aug. 21, 1978 in the name of P. Hochstrate, assigned to applicant's assignee. Some of the patents mentioned above also suggest disposition of the point light source within the light transmitting member by embedding or inserting in a thickened end portion of the light guiding member.
The foregoing constructions all require special construction or manufacturing techniques in order to efficiently distribute light in the light transmitting member.